Place the split peas in a colander. Rinse them well and pick through for any tiny pebbles or debris. In the base of a slow-cooker, combine the peas, onion, carrots, celery, garlic, ham, chicken stock and 1 1/2 cups of water. Tie the sprigs of thyme, parsley and the bay leaf together with a piece of kitchen twine, wrapped around the bundle a couple times, then add to the slow-cooker. Cover and cook on high heat for 4-5 hours, or low heat for 8-10 hours, until the peas are tender and have split to thicken the soup. Season with salt and pepper to taste. Serve topped with chopped parsley.